1. Field of the Invention
This invention relates to moisture-curable urethane-based sealing compositions of a hot-melt type and has particular reference to such a sealing composition for firm attachment of window panes to window frames in the assembly of automotive vehicles.
2. Description of the Prior Art
Different polymer compositions usually called reactive-type hot-melt adhesives are known which suit with coating in a melted state and induce initial adhesion strength upon solidification with temperature drop and subsequently produce final physical quality upon completion of cure reaction. As disclosed for instance in Japanese Patent Laid-Open Publication No. 2-32189, a moisture-curable hot-melt sealant is made up predominantly of a polyether polyol-based urethane prepolymer in which a polyether polyol is used in combination with a polyester polyol. Further, Japanese Patent Laid-Open Publication No. 2-182774 teaches a rapid-curable hot-melt urethane composition which results from blending a polyether polyol having a relatively small molecular weight and a higher glass transition temperature than room temperature with an urethane prepolymer having a relatively large molecular weight and a lower glass transition temperature than room temperature.
The above hot melts are in widespread use in various sectors of industry. In the automobile sector, such type of hot melt is applied as a sealant in fixedly securing window panes to window frames. To this end, the sealant is required to preclude objectionable sagging or running when coated onto a substrate and to develop adequate initial adhesion strength when solidified on cooling, leading to sufficient elasticity and durability when cured completely. The term initial adhesion strength used herein refers to an after-coat period of time required for the sealant to hold the window pane in firmly fixed relation to the window frame. Initial adhesion strength is preferred to be at a level as short as about 10 minutes from the productivity point of view. Sufficient magnitudes of elasticity and durability are needed over a wide range of temperatures and as regards elasticity even at about -40.degree. C., namely at a glass transition temperature of not higher than -40.degree. C., because cars are sometimes used under such adverse conditions in cold territories. Durability is dominated by the resistance to moist heat.
Reactive-type hot-melt adhesives, composed mainly of polyether polyol-based urethane prepolymers, have been found to be too low in cohesiveness before cure to be effective in giving initial adhesion strength. Their inherent physical properties, therefore, will develop depending upon the progress and completion of cure reaction.
The hot-melt sealant of the first-mentioned publication No. 2-32189 is formulated to improve cohesion by the addition of a polyester polyol. Although acceptable in initial adhesion strength, this prior art sealant is less resistant to moist heat due to the polyester segment being liable to become hydrolyzed after cure. Moreover, in addition to its too high a glass transition temperature of about -10.degree. C., the sealant involves low thixotropy prior to cure, thus entailing tedious coating onto a substrate. Thixotropy is a phenomenon in which a viscous liquid shows a decline in viscosity when subjected to mechanical force and on the converse a rise in viscosity when brought to a standstill.
The hot-melt sealant disclosed in the last-mentioned publication No. 2-182774 is designated to laminate film sheets together into an integral structure. This sealant is satisfactory in respect of initial adhesion strength and moist heat resistance but to an extent to invite too high a level of glass transition temperatures, resulting in inadequate elasticity at low temperature.